webadmin: refresh system tree when removing last leaf

- When removing the last element of a list the system
  tree would not refresh. For instance removing the last
  cluster from a data center. This patch fixes that issue.

Change-Id: If9e47bb6cbc5beb4da04b98c8f4b1c38c2932445
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1127600
Signed-off-by: Alexander Wels <awels@redhat.com>